1. Field of the Invention
The invention relates to a piezoelectric actuator, for example for actuating a mechanical component such as a valve or the like.
2. Description of the Prior Art
It is generally known that the so-called piezoelectric effect can be used to produce a piezoelectric element partly comprised of ceramic material with a suitable crystalline structure. When an external electrical voltage is applied, a mechanical reaction of the piezoelectric element occurs, which produces a pressure or tension in a direction that can be predetermined as a function of the crystalline structure and the regions to which the electrical voltage is applied. Such piezoelectric actuators are particularly suitable for use in rapid and precise switching procedures, for example in various injector-equipped gasoline or diesel injection systems for internal combustion engines.
These piezoelectric actuators can be structured in a number of layers, in the form of so-called multilayered piezoelectric actuators in which the layers are respectively interleaved with the electrodes that are used to apply the electrical voltage. To that end, piezoelectric sheets with printed electrode surfaces serving as inner electrodes are stacked in alternation. In this design, a sheet has its connection only on a connection side and on its opposite side, must have an edge without an inner electrode and this edge must be provided with an insulating space. The two sides are then connected on the outside by means of outer electodes. The piezoelectric actuator is thus produced in a known way with a number of plates, much like a capacitor.
These known multilayered piezoelectric actuators predominantly have rectangular inner electrodes with corners that are usually sharply chamfered on opposite sides, on which the connecting potentials are routed to the surface. In this case, the two inner electrode potentials can then be disposed next to each other on the surface, with a ceramic layer between them. These piezoelectric actuators, however, are operated with field intensities that require an insulation of the open leakage paths in the ceramic between the potentials. Suitable lacquers or insulating materials with favorable adhesion and favorable breakdown and insulation characteristics can, for example, be applied in the usual way, by spraying or immersion.
The above-described piezoelectric actuators with inner electrodes routed outward on alternating sides often tend to fail in regions in which the inner electrodes have relatively sharp angles. Since as a rule, the edges on the piezoelectric actuator have to be broken at the mechanically soft piezoelectric ceramic, this yields the above-described corners that are chamfered at an angle of 45°. If inner electrodes protrude outward at these points, then an increased danger of short-circuiting is present there for the reasons mentioned above. Since the electrical field intensity is in inverse quadratic proportion to the curvature radius of the equipotential surface, extremely high field intensities are generated there, which then cause the piezoelectric actuators to fail in this region due to short-circuiting.
It should also be noted that due to physical effects, an insufficient insulation thickness is built up at the sharp edges, i.e. due to a retraction of the insulation lacquer possibly applied. As a result, at the points in which the most insulation is in fact required by the increased field intensity due to the corner effect, the least insulation thickness is actually produced. It would be possible to apply a relatively large amount of lacquer in order to obtain a greater insulation lacquer thickness at the edges, but this can lead to lacquer fractures in the areas in which the lacquer is excessively thick.
DE 199 28 191 A1 has disclosed a multilayered piezoelectric actuator in which the piezoelectric layer that does not have an inner electrode to be contacted is recessed toward the inside in a predetermined region.